Kinetics and mechanism of the base hydrolysis of the cis-chlorobis(ethylenediamine)(imidazole)cobalt(III) cation

Abstract

Base hydrolysis of cis-[CoCl(en)2(imH)]2+(en = ethylenediamine, imH = imidazole) has been studied over the range pH 8.26–9.74 at l= 0.1 mol dm–3(Na[ClO4]) and 25 °C. In this pH range the reaction is first order in the hydroxide-ion concentration (kOH 29 ± 3 dm3 mol–1 s–1). At higher pH (9.89–12.89) the reaction displays less than a first-order dependence on the [OH–] due to conversion into the conjugate base cis-[CoCl(en)2(im)]+(im–= deprotonated imidazole). The pKa for this ionisation is 10.5 at l= 0.1 mol dm–3. At still higher pH the values of kobs. do not become constant, but increase as the pH is increased, consistent with the formation of a second conjugate base by deprotonation of an ethylenediamine ligand. The activation parameters for base hydrolysis at low pH are ΔH‡= 91.2 kJ mol–1 and ΔS‡(298 K)= 89 J K–1 mol–1. The substantial positive ΔS‡ is fully consistent with an SN1(CB) mechanism.